Modern direct injection gasoline engines require fuel injectors to operate under extreme conditions of temperature and pressure and with high fuel pressures. Furthermore, the fuel injector must open and close very rapidly in order to provide multi-pulse injection cycles required for fuel efficiency and low emissions.
Current high pressure direct injection fuel injectors either use inwardly opening valves (nozzle type or multi-hole director) in conjunction with solenoid actuation or outwardly opening valves using piezo-electric actuation. The outwardly opening piezo-electric actuated injector has demonstrated the highest potential for reducing fuel consumption, but the cost of the piezo-stack and driver is prohibitive for high volume applications.
Known outwardly opening piezo-electric actuated fuel injectors generally comprise a valve body having a tip portion defining a spray aperture, a pintle or valve stem extending within the tip portion for axial movement between an extended and a retracted position, the pintle having an external head engageable with the spray aperture to close the spray aperture when the pintle is in its retracted position, a return spring biasing the pintle towards its retracted position, an actuating means in the form of a piezo-stack, acting upon the pintle to urge the pintle to its extended position when the piezo-stack is energised.
The piezo-stack can provide a high opening force to overcome the strong return spring required to hold the valve closed and the high hydraulic forces generated during the high pressure operation of the injector, can provide rapid valve opening and achieves variable valve lift. However, piezo-electric fuel injectors are very costly to produce compared to solenoid actuated injectors and require complex and costly control systems for operation of the piezo-stack.
By contrast, solenoid actuated fuel injectors are much cheaper to produce. However, known solenoid actuated fuel injectors cannot provide the same level of performance as piezo-electric actuated devices, mainly due to the lower opening force achievable by electromagnetic solenoid actuators and the slower rise of force over time. The low opening force of a solenoid renders such unsuitable for high pressure operating outwardly opening injectors because the solenoid cannot overcome the strong return spring required to prevent opening of the valve under the effect of the fuel pressure upstream of the valve, especially during start up when the fuel pressure is lower.